In recent years, along with the spread of personal computers, there are an increasing number of occasions when documents are easily created at home or images are captured with a digital camera or the like.
As a printer used to print these documents or images, inkjet printers are widely spreading, having advantages in low running cost, easy color printing, low operation noise, the capability of printing high-resolution images, and the like.
Particularly, with the resolution of digital cameras increased, the amount (size) of ink droplet discharged has recently become increasingly smaller to enable high-resolution and high-picture quality printing of so-called photo quality. For example, the amount of the ink droplet is as small as 2 pl or less.
When printing high picture quality images with droplets of such small size, to print images of photo quality, it becomes more indispensable than ever before to eliminate a variation in droplet size discharged and stabilize the discharged droplet amount.
For example, with respect to a printhead, manufactured by a semiconductor manufacturing process, for discharging ink using thermal energy generated by an electrothermal transducer such as a heater (heat element), variations in droplet size are mainly divided into three types according to the factors.
Firstly there is a variation ascribable to the manufacturing process of a printhead. Variations within the same substrate, within the same lot and between lots are included in this type. Specifically, the following and other variations are known:                Variation in heater resistance value;        Variation in ON voltage of a switching element controlling the supplying of power to a heater; and        Variation in thickness of a heater protective film.        
Secondly there is a variation ascribable to temperature variation. Specifically, the following and other variations are known:                Variation of ink viscosity;        Variation of heater resistance value; and        Variation of ON voltage of a switching element controlling the supplying of power to a heater.        
Thirdly there is a variation ascribable to the number of concurrently driven printing elements (nozzle or heater) according to printing images. Specifically, the following and other variations are known:                Variation due to voltage drop corresponding to the resistance of common wiring.        
To eliminate variations in droplet size, several methods have hitherto been proposed for coping with each factor of the above variations in droplet.
With respect to the variation ascribable to manufacturing process (the first factor), printing is actually performed while varying the voltage applied to the printhead, and the blurs of printed characters and images are observed to memorize a voltage at which the blurs do not appear. Then when actually used, the printhead is driven with the memorized voltage to thereby ensure that the influence is eliminated.
With respect to the variation ascribable to temperature variation (the second factor), means for detecting a temperature is arranged within the printhead, and according to the detected temperature, driving voltage and driving pulse time are adjusted so as to adjust energy generated by the heater, whereby it is ensured that the influence is eliminated.
With respect to the variation ascribable to the number of concurrently driven printing elements (the third factor), the following three kinds of approaches are known.
According to a first approach, as described in Japanese Patent Publication Laid-Open No. 10-250133, a variable resistance element is inserted between a printhead and a power source unit supplying energy to the printhead, and the resistance value of the variable resistance element is varied according to the number of concurrently driven heaters to thereby stabilize the discharged droplet amount.
FIG. 7 is a view showing a conventional circuit according to the first approach. Shown in the right side of FIG. 7 (portion a in the right side relative to the broken line) is the internal circuit of a printhead, and a portion denoted by reference numeral 71 is a circuit functioning as the variable resistance.
According to a second approach, as described in Japanese Patent Publication Laid-Open No. 2001-58412, a predetermined voltage generation circuit using a series regulator is provided instead of the variable resistance element in the first approach to supply to the heater a voltage according to a control signal.
FIGS. 8A and 8B are views showing a conventional circuit according to the second approach. FIG. 8A shows the internal circuit of a printhead. FIG. 8B shows a configuration of a predetermined voltage generation circuit 81.
According to a third approach, as described in Japanese Patent Publication Laid-Open No. 2001-162801, an error amplifier is operated so that the voltage applied to a heater is equal to a reference voltage, and the driving voltage is controlled so that a switch for driving the heater is changed to the unsaturated operation, whereby a discharged droplet amount is stabilized.
FIG. 9 is a view showing a configuration of a conventional voltage control circuit according to the third approach. Referring to FIG. 9, reference numerals 91 and 92 denote error amplifiers which receive a reference voltage (VREF). Reference numerals 93 and 94 denote a switch and a power transistor, respectively.
As described above, in order to suppress a variation ascribable to the number of concurrently driven printing elements, it is essential to stabilize the voltage applied to the printing element (heater: resistor).
Particularly, in the thermal inkjet technique, stabilizing electric energy applied to the heater has been increasingly desired. According to this technique, a heater, such as a resistor, is used as the printing element, and a pulse signal is supplied to the heater, whereby thermal energy is generated, allowing rapid film boiling to occur on the surface of the heater disposed within a nozzle having filled therein ink. Then the ink within the nozzle is discharged by bubbles generated by thermal energy associated with the boiling.
However, the above-described conventional approaches for eliminating a variation in ink droplet size ascribable to the number of concurrently driven heaters have the following defects.
According to the first approach, as the nozzle density, the number of nozzles, and the printing speed are increased, the current supplied to the printhead becomes larger. In this case, the current value flowing through the variable resistance element can amount to as large as several A (ampere), thus increasing joule loss in the variable resistance element.
Similarly, in a series regulator according to the second approach, also, joule loss increases because energy is collectively supplied to many printing elements. Also, since a feedback control is used, ringing phenomena may occur in response to a sharp voltage variance due to transient response property, thus lacking stability.
According to the third approach, the same number of error amplifiers as the heaters are required. Thus, as the number of printing elements increases, the scale of circuit becomes larger.
In the above description, an inkjet printhead using a heater as the printing element is taken as an example. However, the problem ascribable to a variation in the number of concurrently driven printing elements is common to printheads according to another printing method, as long as printing is performed using a printhead having a plurality of printing elements.